This may be too simple of an answer but here goes,
I have the 38mm su's that were on my L16 when I
had a 510 and I had no trouble with fuel starvation. The inlet was water cooled and has a
cross over equalization tube to help balance the
charge going into the cylinders. I do know that su's can be a pain to figure out but there are a few things that have changed on
Andy's motor that
greatly effect the flow dynamics! It may be as simple as a spark plug that is too hot at 6400rpm
and is pre igniting the now better charge at that
rpm and will show up as crapping out or poping as
if the timing has gone wack-O. The cure is a lower
heat range plug (one or two down),better grade of
gas or as I well know valve bounce,if you havn't
got double springs yet. I just pulled mine apart
and found that all that slop is wearing the top
stem retainer slots badly creating a lip so the valves won't come through the valve guides.
The cure? More work and More money!!
Hope that helps Gotta Go
C-Ya Lime Ricky OOPS! Steve, The Sageman

Before I put the SU's on, I had the 32/36 DGV with the same engine and everything. That carburetor pulled strong to the engines max at about 7,300 rpms. I never experienced any fuel
starvation that I could tell. The SU's kinda die out over 6,500 rpm without changine anything besides the carbs. I think it could possibly be the springs in the SU's, but I am betting
that it has to do more with their design. At 7,300 rpm I might've been experiencing valve bounce, but definitely not at 6,500 rpm. I like the pull of the SU's but a redline at 6,500 rpm
is too soon for me. However, if they can push the engine to 6,500 rpm in 5th gear I won't complain one bit. My Weber only got me to 5,900 rpm in 5th gear :)

-Andy